A Patented tidal stream turbine invented by Aaron Davidson and Craig Hill of Tidal Energy Pty Ltd uses a venturi shaped shroud to increases the turbine efficiency as much as 3.84 times compared to the same turbine without the shroud, making this "a world leading if not world best design".

The venturi not the turbine is the key component of the Davidson-Hill design as the venturi creates a vortex of low pressure behind the turbine - drawing the flow across the turbine. This increased flow allows the turbine to operate at higher efficencies then it would otherwise be capable of without the venturi and producing more power.

The venturi can be designed to fit almost any type of turbine. It can also be integrated or "retro fitted to wind turbines", the company's early commercialisation niche is on fast flowing water currents such as tides, ocean currents, and run-of-the-river.

The technology has a multitude of applications.

It can.

be used to turn a gear box and generator to produce non polluting electricity,

it can be used as a power source to pump water into a header tank,

electrical power can be value added by producing desalinated water or,

by using the electricity it can produce hydrogen and oxygen by the process of electrolysis.

The DHV Turbine can be mounted in several ways depending on the surface conditions the turbine would be likely to encounter.

It can be mounted,

on the sea bed on a mono pile where inclement surface events such as large ocean swells and sea chop might buffet the turbine, (where the turbine is mounted to a monopile in a tidal flow it can be fitted with a turntable so the turbine always faces upstream similar to a wind sock),

or it can be slung under a pontoon on a swing mooring,

or made neutrally bouyant and flown like a kite under water.

In each case power is cabled to shore for use. The simplicity of the design of the DHV allows it to use standard step up gear boxes that are widely available "off the shelf" to suit any size or site application. Use of off the shelf accessories allows D&H to keep costs to a minimum, while focusing on their core technology.

The company is currently in the commercialization stage, with an expected price that competes with coal in the range of 3.5 to 6 cents per kw-h.

With half the Earth’s population living within 50 miles of a coast http://www.whoi.edu/oceanus/viewArticle.do?id=4498, this technology and it's ability to harness the ocean's energy moves the world profoundly toward clean, sustainable and local energy production.

Latest Developments

Davidson-Hill Venturi Turbine History

1996

D&H formed a partnership and began feasibility studies of water currents and venturi technologies capable of harvesting water currents for energy production. Their belief that water has the greatest quantum of renewable sustainable energy on the planet for an energy hungry world drove their quest to seek an energy system that would not hurt or pollute the natural environment.

November 1998

December 1999

D&H tested several sizes of venturis in the Queensland Government Hydraulics Laboratory at Deagon near Brisbane. The tests however, were masked by a bearing failure that produced a diminished output. D&H became convinced that, in future, only the highest standard of materials and equipment would be used in their technology.

2002

The Australian Federal Government Department of Aus-Industry awarded a COMET (Commercialising Emerging Technologies) grant to Tidal Energy Pty Ltd for proof of concept, and scale up testing.

October 2003

D&H achieved a world class if not world record result with the highest efficiency ever achieved from a water current turbine -- 3.84 times (or 384% more energy then the same turbine without the venturi shroud - See report by Dr. Brian Kirke below). Testing was conducted in the waterways of the south east Queensland around the city of the Gold Coast.

March 2004

The Queensland Government EPA awarded Tidal Energy Pty Ltd a QSEIF (Queensland Sustainable Energy Innovation Fund) grant to build the first and largest venturi turbine of its kind in the world and a purpose built vessel to transport the turbine around the South East Queensland regions waterways.

By Aug 2005 the commercial pilot testing was successfully concluded with further improvements to the technology.

How it Works

'Flow is from left to right.'

Two rows of staggered panels - that became known as "cascading hydro panels" - form the venturi geometry.

Counter intuitive the wide angle down stream venturi or shroud resembles a funnel facing the wrong way around. At first glance the wide opening should be facing upstream. The wide end upstream was tested and D&H found that, while it did indeed produce an increase of 10% - 20% it was far short of the performance D&H were looking for. With the wide opening facing upstream into the flow the turbine created a blockage effect when a load was placed on the turbine shaft. This blockage caused the undesired effect where flow went in then out and around the venturi.

Reversing the venturi with the widest section facing down stream and the narrow opening facing into the flow came about as a result of an accident in earlier testing where part of a double ended venturi broke away under a load producing a significant increase in the turbine performance. This wide angle down stream venturi sometimes called a shroud or duct became the focus of future testing.

D&H found that the down stream venturi produced a sub atmosphere of low pressure behind the inlet, augmenting the flow and increasing the flow by 3-4 times as seen by the CFD graphic above. A turbine (whether axial or cross flow) if located/mounted at this location operates at higher efficiencies then the turbine would be capable of, if it were not enclosed in the venturi.

After further refinements using a 2D water flow table, the geometry was tested along with gaps or vents in the sides. It was found that the gaps if spaced at the correct intervals injected flow, from outside flow, into the venturi manintaining the boundary layer flow and reinvigorating the flow behind the turbine and further improving output.

While the figure of 3.84 times is the highest achieved to date, the average was around 3.2 times.

The CFD image above clearly shows the improved flow in red in the throat of the venturi where the turbine is located. Flow is drawn into the venturi from outside the widest point of the inlet with the flow being bent as it is drawn into the venturi.

Several variations of venturi were tested along with axial and vertical axis turbines. The Patent includes round, square and multi sided venturi designs.

Beating BETZ

The Betz Rule states that a turbine in free stream cannot achieve greater than 59.3% efficiency. Shrouded or ducted turbines are not subject to the Betz Rule as is the Davidson-Hill Venturi Turbine (DHV Turbine) not subject to the Betz Rule.

"This has caused quite a stir among engineers who in the early days failed to grasp the concept" said Davidson "with many lost hours agruing over how a turbine could exceed the theoretic limit set by Betz".

Davidson explains, "the turbine does not produce more then 100% of the energy in the flow, rather it increases the velocity and the volume of flow available to the turbine when the turbine is mounted in the venturi. As the average water turbine produces an efficiency of 20% - 25%, trippling this brings the overall efficiency up to 60% - 75%...It is important to note that wind turbines easily achieve efficiencies of 40% - 60% in air, but under water a turbine must push through the 832 times more dense medium of the water. No water turbine that I have heard of has eclipsed the 25% efficiency. Lifting the output of a turbine above 25% became the D&H Holy Grail. The highest turbine efficiency we have achieved to date has been 61%".

"It may be that Betz may not apply to water turbines. There is certainly a lot we still do not understand that some PhD thesis may point out sometime in the future. In any event we are living in exciting times".

Costs

All installations are site dependent but the DHV Turbine can supply power to the grid for as little as US$0.03 - US0.05 cents per kW (depending on the site) and is able to run with a minimum of service due to the simplified design of very few working parts.

Davidson says that the capitalization costs of installing the Davidson-Hill Venturi Turbine would be "comparable to the installation of coal power plants" of similar output. The major difference being that the venturi system "doesn't require costly fuel replenishment".

New innovations in high grade extruded materials and further simplification of the design have reduced the cost of the turbine to little more then the manufacture cost of the materials.

"As the greatest costs in any water current turbine deployment are the costs of physically placing the turbine in the water and not the overall cost of the manufacture and fabrication of the turbine and venture having a system with high output reduces the pay back timeline of capital outlay making the DHV significantly attractive to potential investors as 3-4 open or free stream turbines of similar size would be needed to match the output of just one DHV Turbine".

Advantages

As stated the DHV Turbine is equivalent in output to 3-4 free stream turbines of similar size.

The DHV has greater attraction to investors and to commercial bottom lines.

The turbine is protected from potential damage inside the venturi.

The venturi becomes part of the structural load sharing design to hold the turbine.

The DHV can sit on a turntable or can be slung under a pontoon on a swing moooring allowing it to always point upstream.

Bio fouling can be reduced due to the higher velocity inside the venturi.

Much of the design is similar and can be easily mass produced and flat packed into a shipping container and transported anywhere around the world for final assembly and installation.

It has application in run of river, entrances to bays and rivers in any tidal flow or can be mono-piled to the sea bed in off shore ocean currents and cabled back to the mainland.

The DHV is commercial in flow to slow for open turbines.

The DHV is able to be placed in confined spaces such as seaways where larger turbine are not suitable.

A smaller gear box can be used reducing costs.

Disadvantages

The DHV is a difficult 3D shape that needs to be handled carefully when being deployed.

Specialised marine engineers with an understanding of the deployment/installation proceedures and structural requirements are needed (initiatally) to supervise deployment.

Extra drag loads relative to the increase in velocity the venturi and out put power are produced.

Extra drag loads produced require stronger structures.

Environmentally Benign

The turbines turn slowly at 20-30 revolutions per minute. "Even the sickest guppy could get out of the way," comments Davidson. The Australian fish-net industry has devised under water beepers that can be fitted to alert cetaceans (dolphins and whales etc) away from the nets that turbines can be fitted with. The venturi can also be screened where necessary.

Applications

Run of river, the entrances to bays and rivers, between islands and the mainland and in the open ocean currents.

The DHV is mono directional. Tidal applications require that the device switch directions with the reversal of the flow of the flood and ebb tide. This is accomplished by mounting the turbine on a specially designed turn table so that it pivots with the flow direction and always faces into the flow. "Just like a wind sock at the airport".

The device could also be situated in the out-flow from hydro power plants down stream of dams, harnessing what would otherwise be wasted energy. The technology can also be used to pump water, for desalination, and for hydrogen production (from the electricity generated).

One of the first locations being targeted by the company is a fast-flowing ocean currents. Of considerable interest is Bass Strait off the south coast of Australia, between the island of Tasmania and the mainland. Arrayed underwater similar to an underwater wind farm many turbines could be used to "pump up the grid and supplement the entire southern part of the country" says Davidson.

Testing

The DHV Turbine has been tested and proven over several years.

Third party independent data was collected and results tabled by Dr. Brian Kirke (Former Senior Lecturer at the Faculty of Engineering, Griffith University Gold Coast Campus - Kirke holds a PhD in turbine design)

Commercial scale up was successfully undertaken in 2005.

Results proved the venturi produced a 3 - 4 times improvement in performance compared to the same turbine without the diffuser.

Kirke Reports

Dr. Brian Kirke, PhD Turbine design, is a former Senior lecturer at the Griffith University Gold Coast Campus. He is "arguably the leading expert in water current and tidal turbines" and has published papers in international peer-review journals and Patented a pitching verticle axis turbine.

BEATING THE BETZ LIMIT: MAXIMUM ENERGY EXTRACTION BY A DUCTED MARINE CURRENT TURBINE - A confidential paper, dated 25 Mar 2001, written for the Australian government as they were considering supporting this turbine project (which they did).

Abstract : Unlike conventional hydro and tidal barrage installations, water current turbines in open flow can generate power from flowing water with almost zero environmental impact, over a much wider range of sites than those available for conventional tidal power generation. Recent developments in current turbine design are reviewed and some potential advantages of ducted or “diffuser-augmented" current turbines are explored. These include improved safety, protection from weed growth, increased power output and reduced turbine and gearbox size for a given power output. Ducted turbines are not subject to the so-called Betz limit, which defines an upper limit of 59.3% of the incident kinetic energy that can be converted to shaft power by a single actuator disk turbine in open flow. For ducted turbines the theoretical limit depends on (i) the pressure difference that can be created between duct inlet and outlet, and (ii) the volumetric flow through the duct. These factors in turn depend on the shape of the duct and the ratio of duct area to turbine area. Previous investigations by others have found a theoretical limit for a diffuser-augmented wind turbine of about 3.3 times the Betz limit, and a model diffuseraugmented wind turbine has extracted 4.25 times the power extracted by the same turbine without a diffuser. In the present study, similar principles applied to a water turbine have so far achieved an augmentation factor of 1.7 at an early stage of the investigation.

Download the PDF for more information.

Patents

In 1999, 2001 and again in 2003 the evolving IP and technology being developed was filed with the Australian Patents Office. Earlier Patents have been copied by competitors who make claims of having been awarded Patents, while others have raised millions to commercialise the design that D&H rejected as not good enough years ago".

Profiles

Company: Tidal Energy Pty Ltd

Tidal Energy Pty Ltd is a privately held company, founded in 1999. Their investment model is for investors to invest in the various licensee companies, most of which are expected to become members of a stock exchange.

In developing this technology, engineers from every discipline have been called upon: civil, structural, mechanical, marine, environmental.

D&H plan to go into mass production of the venturi turbine system using high grade marine materials, such as extruded aluminum. The material and fabrication capabilities are abundant on the Gold Coast where the company is located.

The company presently has two production prototypes being beta tested.

They expect that they will be in commercial production by around Q1 2009.

Inventors: Aaron Davidson and Craig Hill

Aaron Davidson and Craig Hill have been working on this concept since 1994.

Mr. Davidson is the co-inventor of several tidal stream turbines. His latest invention (Davidson-Hill Venturi Turbine) holds the world record for the highest efficiency ever achieved by a water current turbine.

Areas of specialty include:

RF radiators.

Venturi Turbines.

Automobile tail pipe diffusers.

He has intimate knowledge of tidal energy technology, patents and commercialization.